A variety of polymeric materials have been used successfully in thin non-oriented (cast) films. A typical film casting process includes the steps of polymer extrusion, melt feeding through a slot die, melt draw-down in the air gap, chill-roll casting, edge-trim slitting, surface treating if necessary, and winding. With the development of faster, more robust winding technologies, cast film line speeds have been increasing in recent years. This increase in line speeds has led to improvements in productivity and manufacturing economics. In this highly competitive segment of the film market, a versatile resin capable of being processed at high line speeds, drawn-down to a thin and uniform web, efficiently quenched to a clear film, and with a good profile of film properties is very desirable.
Polypropylene can be used to make cast film materials having utility in a variety of important commercial applications. The processability and many important end properties of the polymer are closely related to polymer characteristics such as molecular weight, molecular weight distribution (“MWD”), composition distribution (“CD”) and stereoregularity, and these properties in turn are influenced by the catalyst system used to make the polypropylene. Since the introduction of polypropylene in the 1950s, it has been a major interest and trend to manipulate these polymer characteristics to address product needs. Conventional Ziegler-Natta catalysts have long been used to produce isotactic polypropylene. The development of efficient metallocene-based catalyst systems has led to the ability to precisely control polymer characteristics, such as molecular weight, molecular weight distribution and composition distribution over a wide range unattainable by the conventional Ziegler-Natta catalysts.
There remains a need for an improved metallocene catalyzed polypropylene suitable for high output film processing, and supported metallocene catalyst systems capable of efficiently producing polypropylene for cast film applications.